The field of the invention is that of heat-transfer members having selected thermal expansion properties, and the invention relates more particularly to a novel and improved heat-transferring substrate for an electronic circuit and to novel and improved methods for making the substrate.
A commonly used electronic circuit substrate comprises a composite metal having two outer layers of a metal such as copper of relatively high thermal conductivity metallurgically bonded to opposite sides of a core layer of a low expansion metal such as the alloy of 36 percent nickel and the balance iron. That iron alloy is commercially available under the designation Invar. One side of the substrate is typically coated with a thin layer of electrical insulation to support a printed circuit and circuit components in close heat-transfer relation to the substrate. In that arrangement, the substrate not only provides rigid support for the printed circuit, etc. but also provides a heat-sink for withdrawing and dissipating heat from the circuit. The low expansion core of the substrate limits thermal expansion of the substrate to avoid breaking of circuit connections or separation of circuit components from the substrate during thermal cycling of the circuit. However, while the commonly used circuit substrate is highly effective for many circuit applications, the thermal conductivity of the substrate in a direction perpendicular to the layers--the so called Z-axis thermal conductivity--is somewhat limited.
A number of methods for making composite metal circuit substrates having improved Z-axis thermal conductivity have been proposed but thus far have tended to be somewhat ineffective or to be relatively expensive to make. In one proposed process, for example, powders of copper and Invar material are blended together and sintered to form a substrate which has a copper matrix material extending in continuous phase along three mutually perpendicular axes through the substrate. The Invar particles are distributed throughout the copper matrix to limit thermal expansion of the substrate. However, the sintering used for bonding the copper and Invar materials together and for achieving suitable density in the substrate causes diffusion between the copper and iron alloy materials and results in significant loss of thermal conductivity in the copper materials in the substrate. The substrate materials also tend to be relatively expensive to make, particularly in the variety of thicknesses likely to be required for various circuit applications. A proposed method for making such a composite powder metallurgy substrate without such diffusion calls for initially coating the Invar particles with copper to permit use of lower sintering temperatures but again the proposed process tends to be relatively expensive. In another proposed process, silver and Invar powders are sintered to form a substrate without significant loss of thermal conductivity in the silver material but here material costs as well as processing costs are high. Other proposed processes for making such substrates involve perforating a plate of copper or Invar material and then filling the perforations of Invar or copper to provide a desired Z-axis thermal conductivity. Again, however, the process is relatively expensive and flatness is difficult to achieve. It would be desirable if composite metal substrates with improved Z-axis thermal conductivity could be made in a convenient and inexpensive manner.